1. Field of the Invention
The present invention relates to a fuel cell stack comprising a stack body formed by stacking a plurality of electrolyte electrode assemblies and separators alternately in a stacking direction. Each of the electrolyte electrode assemblies includes a pair of electrodes, and an electrolyte interposed between the electrodes. A reactant gas supply passage and a reactant gas discharge passage for at least one reactant gas extend through the stack body in the stacking direction. The fuel cell stack further comprises terminal plates, insulating plates, and end plates provided at opposite ends of the stack body.
2. Description of the Related Art
In general, a polymer electrolyte fuel cell employs a membrane electrode assembly (electrolyte electrode assembly) which includes an anode, a cathode, and an electrolyte membrane (electrolyte) interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. Normally, a predetermined number of membrane electrode assemblies and separators are stacked together alternately to form a fuel cell stack.
In the power generation cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the “hydrogen-containing gas”) is supplied to the anode. A gas chiefly containing oxygen or air (hereinafter also referred to as the “oxygen-containing gas”) is supplied to the cathode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.
In general, an oxygen-containing gas supply passage and a fuel gas supply passage (reactant gas supply passages) extend through the fuel cell stack in the stacking direction for supplying the oxygen-containing gas and the fuel gas as reactant gases to the cathode and the anode. Further, an oxygen-containing gas discharge passage and a fuel gas discharge passage (reactant gas discharge passages) extend through the fuel cell stack in the stacking direction for discharging the fuel gas and the oxygen-containing gas from the cathode and the anode.
In particular, water produced in the reaction in the power generation surfaces of the electrodes tends to flow into the oxygen-containing gas discharge passage, and the water is often retained in the oxygen-containing gas discharge passage. The water produced in the power generation reaction flows into the fuel gas discharge passage by back diffusion, and water condensation occurs in the fuel gas discharge passage. Therefore, the water is also retained in the fuel gas discharge passage.
At the time of starting, or restarting operation of the fuel cell stack, if the temperature of the pipe for supplying the oxygen-containing gas or the fuel gas is decreased, water condensation may occur. Thus, the condensed water flows into the oxygen-containing gas supply passage or the fuel gas supply passage. Further, the oxygen-containing gas and the fuel gas supplied to the fuel cell stack may be humidified beforehand to prevent the electrolyte membrane from being dried. Thus, the oxygen-containing gas discharge passage and the fuel gas discharge passage are narrowed or closed by the retained water. The flows of the oxygen-containing gas and the fuel gas are disturbed, and the power generation performance is degraded.
In an attempt to address the problem, for example, a polymer electrolyte fuel cell disclosed in Japanese Laid-Open Patent Publication No. 10-284096 is known. As shown in FIG. 10, the fuel cell includes a separator 1. An inlet manifold 2a is provided at an upper portion of the separator 1, and an outlet manifold 2b is provided at a lower portion of the separator 1. The inlet manifold 2a and the outlet manifold 2b are connected by a plurality of gas flow grooves 3 extending vertically.
A gas inlet 4a is connected to the inlet manifold 2a, and a gas outlet 4b is connected to the outlet manifold 2b. A branched gas groove 5 is formed in the separator 1. The branched gas groove 5 extends downwardly from the gas inlet 4a, and is curved in the horizontal direction toward the gas outlet 4b. 
When water droplets are mixed with the reactant gas flowing from the gas inlet 4a, the gas droplets are guided by the branched gas groove 5 extending downwardly from the gas inlet 4a. According to the disclosure, in this structure, the water droplets are discharged into the gas outlet 4b. As a result, the amount of water droplets contained in the reactant gas supplied to the gas flow grooves 3 is reduced significantly, and it is possible to prevent degradation of the power generation performance.
However, in the conventional technique, the branched gas groove 5 functioning as a bypass passage is positioned outside the gas flow grooves 3 of the separator 1, i.e., outside the power generation surface. Therefore, the area of the separator 1 cannot be effectively utilized as the power generation surface. Thus, it is not possible to efficiently reduce the overall size of the fuel cell stack.